In order to form a high quality film with ideal interface characteristics, an ultra clean surface must be prepared prior to film growth. An ultra clean surface is one which is characterized as a surface free from: particulates; organic contamination; metallic contamination; natural oxide; molecular adsorption; and surface micro-roughness. It is essential to control the molecular wafer-surface contamination, which has important consequences for the performance and reliability of semiconductor devices, particularly as device dimensions shrink and process temperatures decrease.
In particular, removal of metallic impurities from the surface of silicon is understood in terms of two principal parameters, namely the electron negativity and the heat of oxide formation of the metal. Metals that have higher electron negativity than that of silicon such as Pd, Cu, etc., tend to adhere on the silicon surface by an ion-exchange process. Metals that have a higher heat of metal oxide formation exhibit a tendency to be incorporated into silicon oxide formed during chemical cleaning processes. As a result, very efficient metal cleaning processes can be conducted using HF and H.sub.2 O.sub.2 solutions without causing any degradation in surface micro-roughness.
As conventional cleaning processes for a semiconductor device, such as "SC1" (NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O=1:1:5), "SC2" (HCL:H.sub.2 O.sub.2 :H.sub.2 O=1:1:5), and "SPM" (H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 =4:1), and diluted HF solutions which consist of HF and H.sub.2 O, such are used/performed in a quartz bath supplied with nitrogen gases by means of a bubbling method. When an SC1-only cleaning solution is used for removing metal contamination, in general, the combination of a roughening effect and metal combination in the chemical oxide becomes the main cause of defect related breakdown.
Once silicon is exposed to SC1-mixture solutions, the peroxide will oxidize the silicon surface while the ammonia will etch this chemical oxide away; i.e. a chemical oxide layer will continually form and dissolve as a result of the compensating effect of the two chemical components. Consequently, this slowly etches the silicon. Hence the proportion of peroxide and ammonia in SC1 solutions has to be determined properly.
As the integration of semiconductor devices increases, so has the demand for ultra cleaning processes. To provide such advanced cleaning, various methods are being developed, and a mixture solution of HF and H.sub.2 O.sub.2 has been proposed which is improved from that of HF and H.sub.2 O.
In fabricating a semiconductor device, there are specific kinds of metals, such as Fe, Cu, Ni, Z, etc., which have fatal effects on the characteristics of a circuit element. It typically is impossible to keep contamination under the level of 10.sup.9 atoms/cm.sup.2 to meet the qualification of DRAM's of 64 megabits or greater with conventional methods and cleaning solutions for removal of Fe, Ni and Z.
Cu can be particularly problematic since it is the only metal which when present in an acid HF solution has a higher half-cell potential than hydrogen, and therefore can be deposited on the Si surface from an HF solution. This can be avoided by using ultra pure chemicals or by adding small amounts of H.sub.2 O.sub.2 to the HF solution. Accordingly another method was provided with a cleaning process and mixture solutions consisting of HF and H.sub.2 O.sub.2. Since these mixture solutions produce some particulates with the concentration of 50 ppb and peroxides may be reduced to water and oxygen atoms, it has been difficult to maintain a desirable concentration of peroxides, resulting in poor reproductivity.